1. Field of the Invention
The present invention relates to an optical recording medium having a plurality of information recording layers that can be recorded and read by the irradiation of laser light.
2. Description of the Related Art
Conventionally, optical recording media such as CD-DAs, CD-ROMs, CD-Rs, CD-RWs, DVD-ROMs, DVD-Rs, DVD+/−RWs, and DVD-RAMs have been widely used to view digital moving image contents and to record digital data. In the meantime, the recording capacity demanded of these types of optical recording media has been increasing year by year. To meet this demand, so-called next-generation optical discs, which are capable of recording large volumes of moving images or data, have come into commercial use. In these next-generation optical discs, the wavelength of laser light used for recording and reading has been shortened to 405 nm in order to increase their storage capacity.
In the Blu-ray Disc (BD) standard, being one of the next-generation DVD standards, for example, the numerical aperture of an objective lens is set at 0.85 in order to record and read up to 25 GB on and from a single recording layer.
The volumes of moving images and data are expected to grow ever greater in the future. Accordingly, studies have thus been undertaken regarding a method for increasing the capacity of an optical recording medium by increasing the number of information recording layers included in the optical recording medium. For BD optical recording media, a technique for achieving an ultra high capacity optical recording medium of 200 GB through the provision of six to eight information recording layers has been proposed (I. Ichimura et. al., Appl. Opt, 45, 1794-1803 (2006); K. Mishima et. al., Proc. of SPIE, 6282, 62820I (2006)).
Further to this, another technique has also been proposed which uses an optical recording medium with a part of its information recording layer designated as a read-only area (hereinafter, referred to as a ROM area) and the remaining part designated as a recordable area of write-once or rewritable type. According to this technique, the ROM area can be utilized to record movie or other contents in advance for the purpose of content playback while the remaining area is used as a recordable area in order to achieve a data-recordable optical recording medium.
With the recent trend toward greater volumes, however, the conventional technique of using one part of the information recording layer as a ROM area and the other part as a recordable area has had the problem in that it is difficult to provide sufficient recording capacities for both the ROM area and the recordable area.
It might be possible to adopt an optical recording medium having information recording layers of multilayered configuration so that a recordable layer and a read-only recording layer are individually stacked in order to avoid a reduction in recording capacity. When forming a multilayered optical recording medium by combining information recording layers of different functions, however, it is difficult to maintain high recording and reading quality for each individual information recording layer. To date, a desirable medium configuration has not been disclosed. In particular, optical recording media having a read-only recording layer are used to play movies and other fixed contents, and therefore are often taken out of the recorder player when the playback of the contents finishes. This has led to the problem that fingerprints and other extraneous tend to adhere to the optical recording media, thereby easily causing a drop in recording and reading quality.